This invention relates to air diffusers of the type used in aerating reservoirs and sewage or industrial waste treatment lagoons.
In aerating lagoons using perforated flexible tubing, the practice has generally been to arrange the tubes in a parallel configuration transverse to the direction of flow through the lagoon. In many cases the parallel array of tubing did not cover the entire lagoon, but instead permitted "short circuit" paths to exist which allowed liquid to flow through the lagoon without being aerated. This was especially true in lagoons having sloping side walls. Most lagoons are constructed with flat bottoms and side walls sloping at an angle between 15.degree. to 40.degree. from the horizontal. A diffuser tube cannot be placed partly on a side wall and partly on the bottom, because the hydrostatic pressure difference would cause all or nearly all of the air to be released through the part of the tube on the side wall. Consequently it has been the practice to confine the diffuser tubing to the flat bottom of the lagoon, leaving large triangular short-circuit paths which in some cases approach half the lagoon volume.
In most cases where the parallel array was used, the tubes were relatively long and small in cross-section. They also had air-releasing openings only in their upper parts, and consequently became partially filled with water which had no way to escape. As a result, the air path within the tubes was restricted not only by the small cross-section of the tube, but by the presence of water. If the tube was long, it was considered necessary to feed it with air from both ends by means of headers in order to achieve reasonably uniform bubbling.
One important object of this invention is to provide an air diffusion apparatus which effectively eliminates short-circuiting in an aerated lagoon, one of the major limitations of the old parallel array. This object is accomplished by providing an air manifold, pumping means for supplying air to the air manifold, at least four flexible diffuser tubes, each tube being connected to receive air from the manifold and to release the air in bubble form along substantially its entire length, the connections of said diffuser tubes to the manifold, all being projectable into a horizontal circle having a radius less than approximately one-tenth of the average length of said diffuser tubes, said diffuser tubes extending substantially radially outwardly from the manifold in horizontal directions, and there being at least one of said diffuser tubes in each quadrant. An outward flow pattern induced by the release of air from diffusers so arranged insures complete aeration of an area much larger than that of the diffuser itself, and makes possible the elimination of short-circuiting.
Where the diffuser tubes in the array are so long that the aforementioned restriction of air flow is likely to take place, means are provided at the ends of the diffuser tubes for eliminating water accumulated within the tubes. This means preferably takes the form of a simple slitted diaphragm, the slit being located below the openings in the upper part of the diffuser tube. The means for elimination of water obviates the second header, the necessity for which had, in the past, kept designers from deviating from the usual parallel array.
In its preferred form, the invention utilizes fabric diffuser tubes. Fabric diffusers have been known for their ability to break up air into large numbers of very fine bubbles. Fabric diffusers have been used in the activated sludge process as indicated in U.S. Pat. No. 3,315,895 to Klingbeil et al. However they have not been satisfactorily substituted for flexible perforated plastic diffuser tubes since if made in comparable length, they tend to release bubbles in a very non-uniform manner such that some areas of the fabric tube release bubbles and others do not. It is therefore another object of this invention to provide a fabric diffuser operable in relatively long (e.g. 10 foot) lengths, and therefore suitable for use in lagoon aeration. In order to accomplish this object, the air diffuser preferably comprises an inner conduit connectable to receive air from a manifold, said inner conduit having perforations spaced along its length to release air to its exterior, a foraminous fabric tube surrounding said inner conduit and being of a size such as to provide a space at least between the perforations of said inner conduit and the interior of said fabric tube, means securing the ends of said fabric tube to said inner conduit in order to enclose the ends of said space so that escape of air from the fabric tube is limited to the foramina thereof, and means comprising a quantity of loose pellets filling said space and providing resistance to the flow of air within said space.